Carrying out chemical or biochemical analyses, syntheses or preparations, even at the simplest levels, requires one to perform a large number of separate manipulations on the material components of that analysis, synthesis or preparation, including measuring, aliquoting, transferring, diluting, concentrating, separating, detecting etc.
In developing microfluidic technologies, researchers have sought to miniaturize many of these manipulations and/or to integrate these manipulations within one or a few microscale devices. Many of the above described manipulations easily lend themselves to such miniaturization and integration. For example, the use of these microfluidic technologies has been described in a number of applications, including, e.g., amplification (U.S. Pat. Nos. 5,587,128 and 5,498,392) and separation of nucleic acids (Woolley et al., Proc. Nat'l. Acad. Sci. 91:11348-352 (1994) and hybridization analyses (WO 97/02357 to Anderson).
Despite the application of microfluidic technologies to these manipulations, there are still a number of areas where that application is not so easily made. For example, the performance of large dilutions generally requires the combination of a small volume of the material that is desired to be diluted with a large volume of diluent. By definition, microfluidic systems have extremely small overall volumes, and are typically unable, or less able, to handle the larger volumes required for such dilutions. Further, such large dilutions also typically require the accurate, repeatable dispensing of extremely small volumes of the material to be diluted. However, most microfluidic technology is incapable of accurately dispensing fluid volumes substantially less than a microliter. Although the problems associated with the inability to aliquot extremely small volumes might generally be overcome by performing serial dilutions, such serial dilutions generally require devices with substantially larger volumes, e.g., tens or hundreds of microliters. Specifically, even if one assumes a lower limit of fluid handling of 100 nanoliters, a 1:10 dilution would require a device to handle at least a volume of 1 .mu.l. Further serial dilution steps only increase the required volume.
It would therefor be desirable to provide microfluidic systems which are capable of performing each of the various manipulations required, and which are capable of doing so with a sufficiently small volume whereby, multiple operations can be integrated into a single low volume device or system and performed automatically and with a high degree of precision. Of particular interest would be a microfluidic device or system, as well as methods for using such devices and systems for performing in situ dilution or concentration of a particular material within a microfluidic format. The present invention meets these and many other needs.